Gasification is one method for extracting energy from organic materials. Gasification is a process that converts carbonaceous materials, such as coal, petroleum, biofuel or biomass, into carbon monoxide and hydrogen by reacting a raw material at high temperature with a controlled amount of oxygen and/or steam. The resulting gas mixture is called syngas.
One of the byproducts of gasification is ash (e.g., fly ash). Ash is one of the residues generated during the combustion of char in a gasifier. Fly ash includes the fine particles that rise with flue gases. Ash which does not rise is termed bottom ash. Ash material must be removed from the syngas before it can be used as a fuel. Gasification systems typically use one or more separation methods to remove ash from syngas.
Cyclone separators (cyclones) are used to remove particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. Cyclones can be used to remove some of the ash material from the syngas. However, ash particles having particle diameters less than about 10 μm are not easily removed from a gas stream using cyclones. Due to the small particle size of the ash, the ash is not easily separated from the gas stream and much of the ash exits the cyclone with the gas stream. In gasification systems producing ash particles with diameters less than about 10 μm, additional separation steps are needed.
These gasification systems often employ candle filters. Candle filters are often metallic or ceramic, and each has drawbacks. Metallic candle filters are vulnerable to acid gas corrosion. Sulfur and alkali metal oxy-hydroxides within the syngas stream can form acid gas, which corrodes metal candle filters leading to reduced filter life and frequent filter replacement. Ceramic candle filters are fragile and also susceptible to corrosion. Ceramic candle filters are subjected to high temperatures during separation. These high temperatures can lead to cracks in the ceramic candle filter. Constituents of the syngas stream can also corrode or oxidize the ceramics. Both metal and ceramic candle filters are also vulnerable to inter-pore plugging and failure when the ash particles are submicron (diameters less than 1 μm). Additionally, metal and ceramic candle filters are large and expensive to install, operate and maintain.